1. Field of the Invention
This invention relates to a circuit for controlling a current which is used to drive loads such as light emitting diodes (LED), and more particularly to a circuit for driving LEDs of an LED print head or a thermal head in a printer such as a color printer, a color plotter, or a digital PPC.
2. Description of the Related Art
LED print heads with gradation correcting function are well-known at present. The LED print head comprises LED chip arrays and LED chip driving ICs on a one-to-one ratio or on a one-to-two ratio. The LED chip driving IC controls a current for the LED print head to print one dot (called xe2x80x9cone bitxe2x80x9d hereinafter), thereby controlling and correcting the gradation. For this purpose, the LED driving IC adjusts a voltage at a gate of a transistor which determines the current value of the LED, so that bit correction and gradation control can be performed in the print data.
In other words, the bit correcting data and the gradation controlling data are combined into a group of data by an additional circuit, which is converted into an analog voltage signal by a digital to analog converter, and is then applied to gate terminals of the transistors so as to perform the bit correction and gradation control.
The bit correcting data are used for correcting a current for each dot. Specifically, the data is for fine adjustment of the driving current so that the LED assures printing with a constant density when the same current is applied to all the LEDs. The gradation controlling data are for determining the print density for each dot. The bit correcting data are preliminarily stored in a memory such as a ROM, are then combined with the gradation data to be transmitted for each line, and supplied to the LED driving IC as a group of data.
As described so far, with the conventional LED driving IC, the bit correcting data and the gradation control data are combined beforehand, and are converted into the analog voltage signal by the digital to analog converter. The gate voltage of the transistor is controlled based on the combined data so as to control the LED driving current. For this purpose, an additional circuit is required, which would undesirably make ICs expensive.
It is therefore an object of this invention to provide an LED driving IC which receives bit correcting data and gradation correcting data independently so as to perform bit correction and gradation control without use of an additional circuit therefore.
According to a first aspect of the invention, there is provided a current control circuit for controlling a current for driving printing dot arrays of a print head, comprising: at least one output unit whose output current is adjusted to between 20 and 2nxe2x88x921 times a reference current and which includes xe2x80x9cnxe2x80x9d output transistors, to a gate terminal of which different voltages are independently applied; at least one bit corrector for correcting bits of print data by performing full-scale control of a voltage applied to gate terminals of the output transistors according to stored correction data; and at least one gradation controller for controlling according to m-bit gradation data a voltage applied to drain and source terminals of the output transistors to a voltage of between 20 and 2nxe2x88x921 times a reference voltage in a range of the output transistors.
With this arrangement, the current for driving printing dot arrays of a print head is controlled by using the range of the output characteristics of the output transistors. The digital to analog converter of the gradation control circuit is designed to apply an analog voltage of 2m gradation to the drain and source terminals of n output transistors. Therefore, the print data can be subject to the gradation control substantially in proportion to the analog output from the digital to analog converter as specified by the gradation data. Furthermore, bit correction can be performed by applying an output voltage to gate terminals of the output transistors from the bit correction circuit and by carrying out full-scale control of the voltage applied the output transistors. Thus, bit correction can be performed according to the correction data without use of a special synthesizing circuit.
The bit corrector comprises n-bit shift registers, and activation and deactivation of the output registers is determined according to values stored in the shift registers.
The gradation control comprises m-bit shift registers, and each of the m-bit D/A converters converts m-bit data to an analog output.
A voltage between a source terminal and a drain terminal of the output transistor is controlled by the output of the digital to analog converters.
The output units, correctors and gradation controllers are provided in equal plural numbers. The correctors are connected in series so as to serially receive predetermined correction data.
According to a second aspect of the invention, there is provided a printer with an LED print head, comprising: at least one output unit whose output voltage is adjusted to 20 to 2nxe2x88x921 times a reference current and which includes n output transistors, to a gate terminal of which different voltages are independently applied; at least one bit corrector for correcting bits of print data by performing full-scale control of a voltage applied to gate terminals of the output transistors according to stored correction data; at least one gradation controller for controlling according to m-bit gradation data a voltage applied to a drain terminal and a source terminal of the output transistors to a voltage of 20 to 2nxe2x88x921 times a reference voltage in a range of the output transistors by using m-bit gradation data; and at least one LED array including a plurality of LEDs to receive current from a plurality of output units, and emit light.